Inertial dimensional metrology

ABSTRACT

A method of performing dimensional metrology of an object ( 12 ) includes incorporating an Inertial Measurement Unit (IMU- 18 ) with an elongate probe ( 20 ) in a portable metroprobe ( 10 ). A tip ( 22 ) of the probe ( 20 ) has an offset length (L) from an origin ( 26 ) of a coordinate system in the IMU ( 18 ) and position (X,Y,Z) thereof is correlated based on attitude (A,B,C) measurement of the IMU ( 18 ). The metroprobe ( 10 ) is transported in sequence to a complement of survey points (Pn) on the object ( 12 ) for measuring corresponding coordinates (X,Y,Z) thereof based on measured attitude (A,B,C) of the IMU ( 18 ).

BACKGROUND OF THE INVENTION

The present invention relates generally to dimensional metrology and,more specifically, to large volume physical measurement of threedimensional (3D) objects.

Dimensional Metrology is the science of calibrating and using physicalmeasurement equipment to quantify the physical size of, or distancefrom, any given object. Inspection is a critical step in productdevelopment and quality control.

Dimensional Metrology requires the use of a variety of physical scalesto determine dimensions, with the most accurate of these beingholographic etalons or laser interferometers. The realization ofdimensions using these accurate scale technologies is the end goal ofdimensional metrologists.

Modern measurement equipment include hand tools, Coordinate-MeasurementMachines (CMMs), machine vision systems, laser trackers, and opticalcomparators. A CMM is based on CNC technology to automate measurement ofCartesian coordinates using a touch probe, contact scanning probe, ornon-contact sensor.

Optical comparators are used when physically touching the part isundesirable. Optical comparators can now build 3D models of a scannedpart and internal passages using x-ray technology.

Furthermore, optical 3D laser scanners are becoming more and common. Byusing a light sensitive detector (e.g. digital camera) and a lightsource (laser, line projector) the triangulation principle is employedto generate 3D data, which is evaluated in order to compare the measuresagainst nominal geometries either in a scale drawing or CAD Model.

In some cases, the object to be measured is transported to an area withstationary measuring devices for measurement. Typically in large volumedimensional metrology, portable measuring devices are transported to thelarge object for measurement.

Large volume metrology examples include precision measurement ofaircraft and spacecraft, energy generation structures and devices, andlarge manufacturing and assembly facilities.

The CMM is a very powerful measuring device used in dimensionalmetrology because it simultaneously produces coordinates of a point onthe object being measured based on a reference location of the CMM usinga suitable coordinate system like the three orthogonal axis Cartesiancoordinates X, Y, and Z having a common reference origin.

The laser tracker is a popular portable CMM that can calculate X,Y,Zcoordinates for any point on an object. This is accomplished bymeasuring the distance between the tracker and each target point with alaser and combining it with the horizontal and vertical angles of thelaser pointing device embodied in the tracker using a common referencecoordinate systems for all points in the measurement survey.

An optical target in the exemplary form of a Spherically MountedRetro-reflector (SMR) is placed at the desired point on the object forthe laser tracker to precisely determine laser range and fix horizontaland vertical angles of the emitted laser beam in the pointing device.

Other portable CMMs include theodolites, robotic total stations, and asystem of camera photos called photogrammetry. They all require Line ofSite (LOS) between the portable CMM and the target point on the objectthey are measuring.

Since ultimately all the desired points measured on the 3D object needto be plotted in their exact relationship with each other in a suitablecoordinate system, and because the CMM will most likely not havevisibility on all the desired points from one location, the LOSrequirement becomes a significant problem.

In large volume metrology, the object being surveyed is typically largein three dimensions and typically complex in configuration, and maytherefore include a significant number of recessed or obstructed targetpoints hidden from LOS view of the CMM within the full complement ofdesired survey locations or points.

However, because this type of CMM is portable, the CMM can be relocatedto a new LOS reference location, or a second CMM may be used, forproviding LOS measurements of survey points previously hidden at thefirst CMM location. CMM measurements from both viewing or sourcelocations will therefore include both survey points with LOS coordinatemeasurements thereof, and other survey points hidden from LOS view ofthe differently located CMMs.

Since the two CMM viewing locations will have different coordinatereferences, a mathematical work-around to the LOS requirement, such asleast squares optimization, may be used to mathematically tie togetherthe measured coordinates based on some of the common survey pointshaving LOS visibility from both CMM viewing locations to establish acommon coordinate reference system for all measured points from bothviewing locations.

Other solutions for measuring hidden points lacking LOS visibilityinclude special optical targets cooperating with the CMM that includetouch probes that can reach the hidden points while at least someportion of the probe remains within LOS visibility of the CMM.

However, such optical targets probes can have various configurationsincluding different benefits and different problems in measuring thehidden survey point.

Significant to large scale dimensional metrology is the typicalrequirement for precision measurement of the 3D object coordinatelocations X,Y,Z within very small dimensional tolerances of aboutplus/minus 0.6 mils (0.0006 inches or 15 microns), for example.

The typical laser tracker CMM can achieve this high precision; andhighly specialized optical targets may be used therewith for matchingsuch high precision based on different technologies having differentproblems and different benefits, and at correspondingly different cost.

Various optical targets and probes are known for various fields ofendeavor including land surveying, and vary substantially inconfiguration and operation, with correspondingly different accuracy ofmeasurement.

Fundamental to metrology are the typical six degrees of freedom (DOF)associated with 3D objects, which can be measured in a suitablecoordinate system such as the exemplary six-axis Cartesian coordinatesystem introduced above. Three orthogonal linear axes X, Y, and Z extendoutwardly from a common origin for defining linear position therefrom;and three angular or rotary axes A, B, C define angular position orattitude around the corresponding linear axes, commonly known as roll,pitch, and yaw.

Various technologies are commonly known for measuring linear positionand angular attitude with varying degrees of complexity and accuracy.And, such various technologies may be combined in various manners forvarious benefits.

Many common measuring technologies are based on optical measurementshaving various optical encoders or camera systems, which require LOS.Other technologies include the Global Positioning Satellite (GPS) systemcommonly used in navigation for measuring or determining location basedon longitude and latitude positions, but subject to the substantialproblem of GPS signal loss.

Still other technologies include the Inertial Measurement Unit (IMU)also commonly used in navigation in which cooperating accelerometers andgyroscopes measure relative movement of the IMU in the six DOF, butsubject to the also significant problem of inherent temporal drifterrors.

All such measuring technologies have different capabilities anddifferent problems, and correspondingly different costs.

For example, fundamental to IMUs is the significant drift errorsinherent therein which increase exponentially, or quadratically, withtime. Accordingly, commercial inertial sensors based on IMUs have asix-order magnitude difference in price and performance in differentconfigurations or grades thereof.

Four IMU grades include automotive & consumer; industrial; tactical; andmarine & navigation having correspondingly decreasing drift errorsresulting in horizontal position errors of about 7900 km/hr, 190 km/hr,19 km/hr, and 1.6 km/hr, respectively, with cost ranging from low forconsumer grade to exceedingly high for the marine grade.

However, one particular advantage of IMUs is their dead-reckoningcapability to measure both linear and angular positions without regardto the loss of LOS or GPS signal problems. Another particular advantageof IMUs is modern advancements thereto in which the size, cost, anddrift errors of IMUs continue to decrease.

Accordingly, one object of the present invention is to provide improvedlarge volume dimensional metrology of an object.

Another object of the invention is to provide an improved method formeasuring location of one or more of the full complement of surveypoints having blocked LOS in a measurement survey of the object.

Another object of the invention is to provide location measurement ofthe hidden point with preferential precision thereof.

Another object of the invention is to provide an improved method andsystem for conducting large volume dimensional metrology having reducedcomplexity and cost.

BRIEF DESCRIPTION OF THE INVENTION

A method of performing dimensional metrology of an object includesincorporating an Inertial Measurement Unit (IMU) with an elongate probein a portable metroprobe. A tip of the probe has an offset length froman origin of a coordinate system in the IMU and position thereof iscorrelated based on attitude measurement of the IMU. The metroprobe istransported in sequence to a complement of survey points on the objectfor measuring corresponding coordinates thereof based on measuredattitude of the IMU.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, in accordance with preferred and exemplary embodiments,together with further objects and advantages thereof, is moreparticularly described in the following detailed description taken inconjunction with the accompanying drawings in which:

FIG. 1 is an elevational isometric view of a survey system using aninertial metroprobe for performing large volume dimensional metrology ona large object.

FIG. 2 is a schematic view of the metrology system shown in FIG. 1 forconducting a coordinate measurement survey of a set of points on theobject.

FIG. 3 is a flowchart for performing the metrology survey of FIGS. 1 and2.

FIG. 4 is an elevational isometric view of a modified metrology systemadditionally including a laser tracker cooperating with the metroprobeshown in FIGS. 1 and 2 for conducting the measurement survey of theobject shown in FIG. 1.

FIG. 5 is a schematic view of the modified metrology system shown inFIG. 4 for conducting the measurement survey of the set of points.

FIG. 6 is a flowchart for performing the metrology survey of FIGS. 4 and5.

DETAILED DESCRIPTION OF THE INVENTION

Illustrated in FIG. 1 is a metrology probe, or metroprobe, 10specifically configured for conducting large volume dimensionalmetrology of an object 12, such as the exemplary cargo aircraft.

In large volume dimensional metrology, a preferential complement or setof measurement points Pn are suitably selected on the object, and mayhave any desired quantity or number, with n ranging in value from 1, 2,3, . . . to N, where N is the total number of measurement or surveypoints Pn desired. The survey points Pn correspond with various pointlocations on the object 12 for which precise coordinate locationsthereof are desired.

The metroprobe 10 may be controlled and functionally operated by using asuitable controller 14, such as a digitally programmable laptopcomputer, operatively joined thereto by either an electrical cord or bywireless communication using a standard Wireless Local Area Network(WLAN) having suitable WLAN adapters in both devices.

The metroprobe 10 includes a suitable housing or body 16 in which arestructurally supported or integrated an Inertial Measurement Unit (IMU)18 and a removable probe 20. The probe 20 is affixed to the bottom endof the housing 16 in any suitable manner such as pin & socket or bayonetmounting for joining and removing the probe as desired.

The metroprobe assembly 10 is relatively small and portable, and theprobe 20 may have any suitable configuration and length as desired forextending the reach of the metroprobe 10 in conducting dimensionalmetrology. The probe 20 has a small spherical tip 22 at the distal,bottom end thereof for use in contacting or touching any of the surveypoints Pn during the measurement survey.

The dimensional metrology measurement survey is illustratedschematically in FIG. 1 in which the metroprobe 10 is manuallytransported by metrologist or user conducting the survey to the varioussurvey locations or points Pn in any desired sequence 1, 2, 3, 4, . . .N so that the probe tip 22 may be temporarily placed in contact with thedesired survey point Pn for measuring or recording its 3D position orlocation in space based on a suitable coordinate system and suitablereference location.

The line-of-sight (LOS) of some of the survey points may be blocked byobstructions in or around the object 12, which points are therebyhidden, as shown for the exemplary survey point P4 hidden behind astructural rib of the aircraft. The user may therefore simply carry themetroprobe 10 to each survey point to obtain direct access thereto bythe probe tip 22, as long as the slender probe 20 is able to reach thedesired survey point, such as the otherwise hidden point P4.

The metroprobe 10 is illustrated in more detail in FIG. 2 and mayinclude any suitable commercially available IMU 18 as desired, withcorresponding size, performance, and cost, from relatively low to high.

For example, one suitable navigation-grade IMU is commercially availablefrom distributors for Honeywell Aerospace, Phoenix, Ariz., in modelHG9900 IMU having correspondingly high performance and cost.

Another suitable example is the industrial-grade VN-100 IMU commerciallyavailable from distributors for VectorNav Technologies, Dallas, Tex.,which uses Micro Electro-Mechanical System (MEMS) sensor technology tointegrate various sensors and cooperating IMU processor in a small formfactor.

The IMU 18 is shown schematically to include three linear accelerometers24 arranged orthogonally to each other to correspond with the threeorthogonal linear axes X,Y,Z of the conventional Cartesian coordinatesystem having a common origin 26, which may be the center-of-gravity ofthe module defining the IMU 18.

The IMU 18 also includes three gyroscopes, or gyros, 28 arrangedcoaxially with the corresponding accelerometers 24 to correspond withthree rotary or angular axes A,B,C of the Cartesian coordinate system,for measuring roll, pitch, and yaw, respectively.

Accordingly, the three accelerometers may be designated 24 x, 24 y, 24 zto correspond with the three linear axes X,Y,Z along which they measurelinear or translation movement of the IMU; and the three gyros may bedesignated 28 a, 28 b, 28 c to correspondingly measure angular or rotarymovement of the IMU around the three linear axes X,Y,Z.

The basic IMU 18 may include any other conventional features foroperating in a stand-alone module as typically commercially available,including for example its own internal digital processor for controllingoperation thereof, and having suitable or standard input and outputports for communicating with an external computer, such as thecontroller 14.

In the exemplary VectorNav configuration identified above, the IMU mayalso include 3-axis magnetic sensors, a barometric sensor, and atemperature sensor cooperating with the basic three accelerometers andthree gyroscopes, all operatively joined to a 32-bit microprocessor andmemory device.

In basic conventional operation, the IMU 18 utilizes the threeaccelerometers 24 and three gyroscopes 28 to produce a three dimensionalmeasurement of both specific force and angular rate or velocity.Specific force is a measure of acceleration relative to free-fall, andangular rate is a measure of rate of rotation.

Subtracting the gravitational acceleration in the IMU 18 results in ameasurement of actual coordinate acceleration. And, by providing the IMUwith a reference position, the IMU may thereafter compute its ownposition and velocity by mathematically integrating the linearaccelerations as measured by the three accelerometers 24 suitablycorrected using the angular velocities as measured by the threegyroscopes 28 in a conventional manner.

The ability of the IMU to measure and calculate its own position in 3Dspace is dependent firstly on the inherent accuracy of theaccelerometers and gyroscopes themselves, as well as the computationalaccuracy of the mathematical processing of the data measured thereby.Calculated position accuracy is also dependent on filtering out commonerror sources, such as sensitivity to supply voltage variations andtemperature dependent hysteresis.

A conventional IMU is typically calibrated over a preferred operatingtemperature range to determine bias, sensitivity, and cross-axisalignment of each individual component; and corresponding calibrationcoefficients are stored in the IMU for use during operation in filteringout the common error sources. In this way, the basic IMU 18 can measureits own coordinate location or position using the combined outputs fromits accelerometers and gyros, with an accuracy and drift error asspecified for the particular make and model of the IMU commerciallyavailable at a corresponding price.

However, understanding the different components of the inertial IMU anddifferent performance thereof may be used to advantage in speciallyconfiguring the inertial metroprobe 10 for enhanced operation andutility in large volume dimensional metrology in contradistinction fromthe typical use of IMUs for ordinary inertial navigation use.

In navigation, location on the globe is desired, and is typicallyrepresented by latitude and longitude in a substantially planarenvironment represented by the typical surface map of the globe.

In dimensional metrology, 3D locations of the 3D object are beingmeasured, with typically higher precision than needed for commonnavigation.

And as mentioned above, an understanding of the time-dependent, ortemporal, drift errors of IMUs can be used to advantage in performingdimensional metrology.

Drift error is a general term representing the many errors inherent inthe typical IMU based in large part on the mathematical doubleintegration required for the three accelerometers to determine lineartranslation.

An IMU starts operation at an initial location and initial time, andthen measures linear acceleration along the three linear axes X,Y,Z andangular velocity along the three rotary axes A,B,C. By mathematicallyintegrating the measured acceleration over time, velocity can beobtained, and by further integrating the velocity, displacement orlinear movement or translation along the three linear axes X,Y,Z canalso be obtained.

In this manner, the IMU operates by dead reckoning from a known startinglocation by measuring linear movement therefrom along the three linearaxes X,Y,Z to the present location of the IMU. Various drift errorsaccumulate over time in calculating the location of the IMU as it movesin space, which errors may increase exponentially, or quadratically,over time.

As indicated above, such drift errors can accumulate so that the presentlocation of the IMU may be incorrect after an exemplary hour of travelby 1.6 km to about 7900 km depending on the grade of IMU.

Of course, such position errors would be unacceptable where higherprecision is appropriate, and therefore various techniques can be usedto reduce or accommodate drift errors in an IMU.

One conventional example for accommodating drift errors in an IMU is theintegration therewith of a GPS device to provide an external measurementof position, subject to the inherent positional errors of GPS, at acorresponding increase in complexity and cost.

However, the errors in an IMU are different between the gyroscopes andaccelerometers, with gyro errors being substantially less thanaccelerometer errors due to their different configuration and operationin the IMU. This difference is typically specified as Gyro Bias Error asdistinct from Accelerometer Bias Error in specifications presented forcommercial IMUs.

Accordingly, the probe 20 illustrated in FIG. 2 is structurallyintegrated with the IMU 18 in a suitable configuration so that the probetip 22 can be fixed relative to the IMU 18, and suitably correlated tothe origin 26 of the six-axis Cartesian coordinate system X,Y,Z,A,B,C.

For example, the probe 20 has a first offset length L measured from theorigin 26 to the bottom of the distal probe tip 22. The tip 22 itselfmay have a spherical configuration like a typical touch probe, with asuitably small radius R.

The probe 20 is coaxially aligned with the vertical Z-axis of the IMU 18in the exemplary configuration shown in FIG. 2, or may have any otherorientation as desired.

The particular orientation and offset length of the probe 20 istherefore fixed and known relative to the IMU coordinate system so thatlocation and angular orientation or attitude of the IMU correspondinglycontrols location and attitude of the fixedly attached probe 20.

By correlating position of the probe tip 22 to the origin 26 of the IMU18, any change in location and angular attitude of the IMU 18corresponds directly with location and attitude of the affixed probe 20,and its tip 22 in particular.

In this way, a basic method of performing dimensional metrology of theobject 12 includes the simple correlation in position of the probe tip22 having the first offset length L from the origin 26 of IMU coordinatesystem based on attitude measurement of the IMU itself.

The metroprobe 10 is merely hand-carried or transported by the userduring the survey from an initial reference location, such as P1 forexample, in a suitable sequence to simply touch or directly contact eachof the desired survey points Pn on the object 12 for inertiallymeasuring corresponding coordinates X,Y,Z of the survey points Pn basedin part on measured attitude of the affixed IMU 18, which attitude ismeasured by the gyros 28 of the IMU 18 itself.

A suitable record button 30 is provided in the metroprobe 10 foractuation by the user to record in the controller 14 the specificlocation and attitude of the metroprobe 10 when the probe tip 22contacts each desired survey point.

The IMU 18 then measures its own linear travel or translation along thethree axes X,Y,Z, to the recorded survey point, which linear travel isidentical to the linear travel or translation of the probe 20 and itstip 22.

Quite significantly, the metroprobe 10 can also rotate in 3D sphericalspace, about the origin 26 for example, to have any orientation orattitude within the full 360 degrees of rotation along the three rotaryaxes A,B,C.

In FIG. 2, the metroprobe 10 is shown at the left in an exemplaryvertical attitude, with the IMU 18 positioned vertically atop thecoaxial probe 20 at the bottom end thereof. Full rotary movement of themetroprobe 10 allows infinite attitudes in space from IMU-side up, orupside-down with the probe 20 up and the IMU 18 down, or at any attitudetherebetween. The metroprobe 10 is therefore operable without attitudelimitation, including full attitude motion from horizontal to vertical,and all inclination attitudes therebetween.

The three gyros 28 accurately measure angular rate of rotation orvelocity, which may be mathematically integrated in the IMU processor toaccurately calculate, or measure, angular orientation or attitude of theIMU 18, with the attached probe 20 experiencing the same angularmovement and attitude.

Accordingly, the position of the probe tip 22 may be mathematicallyestablished by combining the linear translation and angular attitude ofthe probe 20 as it travels with the metroprobe 10 in any suitableorientation for accessing any survey point, including otherwise hiddensurvey points.

Simple trigonometry is used to resolve the three components of theoffset length L of the probe 20 along the three linear axes X,Y,Z whichare then added to the measured coordinates X,Y,Z of the IMU 18 at itsorigin 26 to correspondingly establish the linear position andcoordinates X,Y,Z of the probe tip 22 itself.

For example, the linear coordinates of the probe tip 22 may be expressedas P22(XYZ)=P26(XYZ)+V22(XYZ); where V22 represents the coordinatevector from the origin 26 to the probe tip 22.

The tip vector V22 can be resolved by trigonometry for obtaining therespective components of the probe length L from the origin 26 asrepresented by L(XYZ) which is a function of the attitude A,B,C of theprobe 20.

In the special configuration of the coaxially aligned IMU 18 and probe20, changes in attitude in the yaw C-axis do not affect the coordinatesof the probe tip 22 and simplifies the trigonometry.

For the exemplary vertical attitude A,B,C=(0,0,0) of the metroprobe 10shown at the left in FIG. 2, the coordinates P22(XYZ) of the probe tip22 are simply the measured coordinates P26 of the IMU origin 26 minusthe offset length L, or P22(XYZ)=(X,Y,Z−L), which represents the linearcoordinates of measured first survey point P1.

In another exemplary vertical attitude of the metroprobe 10 shown atsurvey point P4, the metroprobe 10 has an attitude inclination angle dB,which is the differential or delta angle in the −B pitch direction only,i.e. (A,B,C)=(0,−dB,0). The −dB attitude angle inclines the probe 20counterclockwise solely in the X-Z plane, and correlates with thecoordinate position of the probe tip P22(XYZ)=(X+L×Sine(dB), Y,Z−L×Cosine(dB)) relative to the new P26(XYZ) coordinate position of theIMU origin 26 from which the probe contacts the fourth survey point P4whose measured coordinates equal the new P22(XYZ) coordinates.

The vector V22 may be similarly resolved in the YZ plane, or in anyattitude corresponding with the changing attitude of the metroprobe 10during operation.

Since the metroprobe 10 can be hand-held and properly operate at anyangular attitude, it is not constrained in use and may be freely movedto access any survey point without regard to LOS obstructions as long assuitable access is provided to the probe 20 itself.

Accordingly, the probe 20 can be removable and replaced with variouscustom configurations for accessing any desired survey points requiringshort or long lengths, or straight or curved paths, with the probe tip22 nevertheless being simply correlated to the IMU origin 26 forestablishing its linear X,Y,Z coordinate position relative thereto.

Because the IMU 18 includes the three accelerometers 24 and three gyros28 for correspondingly defining the three orthogonal linear axes X,Y,Zand three respective angular axes A,B,C, the linear coordinate positionX,Y,Z of the probe tip 22 can be readily correlated to the CartesianX,Y,Z coordinate system and its origin 26 based on the angular attitudeA,B,C of the IMU 18 as measured by its own gyros 28.

And, as indicated above, gyro bias errors are substantially small inconventional IMUs, and much smaller than accelerometer bias errors, andtherefore allow increased accuracy in measuring coordinate position ofthe probe tip 22 in the specially configured metroprobe 10.

FIG. 3 shows a flowchart depiction of the basic method of performinglarge volume dimensional metrology using the dedicated inertialmetroprobe 10 in a relatively simple configuration integrating theconventional IMU 18 with the suitable probe 20 and controlled by asuitable controller 14 in the simple form of the typical laptop computerconfigured with suitable control and measurement software.

Since the typical IMU measures relative movement of the IMU itself, themeasurement survey preferably begins by initially transporting themetroprobe 10 to any suitable reference location to establish at theprobe tip 22 three linear reference coordinates relative to the threelinear axes X,Y,Z and three angular reference coordinates relative tothe three angular axes A,B,C, all based on the reference origin 26 inthe six-axis Cartesian reference coordinate system.

For example, the first survey point P1 itself may be used as the firstreference location to establish nominal X,Y,Z reference coordinates suchas (0,0,0) for the IMU or coordinate system origin 26, or for the probetip 22 itself as desired. This may also establish the nominal A,B,Creference coordinates such as (0,0,0) for the attitude of the IMU 18 orprobe 20 as well.

Accordingly, as the metroprobe 10 is hand-carried in series to theremaining survey points Pn, the respective linear X,Y,Z coordinatesthereof may be measured by the metroprobe 10 relative to the (0,0,0)reference coordinates of the reference location of the metroprobe 10.

With the linear coordinate position X,Y,Z of the probe tip 22 correlatedto the attitude of the IMU 18 as described above, transport of themetroprobe and re-orientation thereof to reach subsequent survey pointswill readily establish and record corresponding or correlated linearcoordinates X,Y,Z at each survey point Pn upon simply depressing therecord button 30 on the metroprobe 10.

Since the probe tip 22 may have any suitable offset position in 3Dspace, its position is preferably correlated to the coordinate systemorigin 26 based on corresponding angular position or attitude of theprobe 20 relative to the three linear axes X,Y,Z. The probe tip 22 maybe offset based on any one, or more, of the three axes X or Y or Z, butin all cases simply trigonometry can correlate the linear coordinates ofthe tip 22 in the IMU coordinate system.

In the simple configuration shown in FIG. 2, the probe 20 is coaxiallyaligned solely with the Z-axis in the X-Z plane, and its tip 22 has asingle offset from the origin 26 measuring L in length, without anyoffset of the tip along the X & Y axes.

In the exemplary method illustrated in FIGS. 1-3, the metroprobe 10,alone, is transported by the user in a sequential survey of the multiplesurvey points Pn, at which the linear positions X,Y,Z of the IMU 18 aremeasured and recorded by depressing the record button 30.

Simultaneously, attitude of the IMU 18 in the angular axes A,B,C is alsorecorded and used to correlate therewith the corresponding attitude ofthe probe 20.

This survey is recorded and controlled by the laptop computer 14 whichis used to establish or calculate the corresponding linear coordinatesX,Y,Z of the probe tip 22 in contact with the survey points Pn ascorrelated with the measured linear positions X,Y,Z of the IMU 18.

Since the IMU 18 includes the accelerometers and gyroscopes which definethe corresponding three orthogonal linear axes and three respectiveangular axes, the angular attitude A,B,C of the IMU 18 in three axes ismeasured by the gyros 28 in the IMU 18, and the linear positions X,Y,Zof the IMU 18 in three axes are measured by both the threeaccelerometers 24 and three gyroscopes 28 in a conventional manner.

In a preferred and basic embodiment, the metroprobe 10 is autonomous inperforming the dimensional metrology of the object 12, and isoperatively joined to the controller 14 for establishing the linearcoordinates X,Y,Z of the probe tip 22 at the many survey points Pnrelative to the three orthogonal linear axes X,Y,Z based on the measuredthree-axis linear position X,Y,Z and three-axis attitude A,B,C of theIMU 18.

Precision or accuracy of the measured coordinate locations X,Y,Z foreach of the full complement of survey points Pn is therefore basedsolely on the inherent accuracy of the conventional IMU used in themetroprobe 10, based in part on the bias errors of the accelerators andbased in additional part on the bias errors in the gyros.

However, as indicated above, the gyro bias errors are substantiallysmaller than the accelerometer bias errors which improves the overallprecision and accuracy of the metroprobe 10 in its special configurationfor conducting large volume dimensional metrology in the exemplarymethod disclosed above.

Since the IMU 18 is subject to temporal drift errors, the linearcoordinates X,Y,Z at the survey points Pn can be suitably corrected toreduce the drift errors.

For example, the method may be modified to include transporting themetroprobe 10 between two suitable reference locations in surveying thesurvey points Pn, and establishing a position error vector 32 as shownin FIG. 2 based on a difference in linear positions X,Y,Z measured atthe two reference locations, and then mathematically resolving the errorvector to reduce corresponding errors in the linear coordinates X,Y,Z atthe applicable survey points Pn.

FIG. 2 illustrates schematically how the IMU drift error increases withtime during transportation of the metroprobe 10 during the measurementsurvey. Since the survey can take minutes to hours to complete dependingon the particular survey, the accumulation of drift error can be smallor large, especially since the accelerometer drift error can increaseexponentially with time.

In FIG. 2, the drift error increases in time for each of the threeaccelerometers 24, and collectively result in the total drift errorvector 32 which has increased size at subsequent survey points.

The total error in the linear coordinates X,Y,Z can be represented bythe single error vector 32 at any suitable survey point and can bemathematically resolved into constituent components in the three linearaxes X,Y,Z, but then requires suitable distribution or attribution toall previous survey points from which it was made.

Since the drift error is known to accumulate exponentially according toIMU performance, knowledge of that IMU performance can be suitably usedto correspondingly reduce more error in the linear coordinates atsubsequent survey points.

In other words, the drift error accumulates according to knownperformance of the IMU, and therefore can be resolved and distributed inreverse sequence over the relevant time period.

Since the first survey point, P1 for example, initiates the process atthe reference coordinates (0,0,0) it establishes a reference zero drifterror condition.

If the metroprobe 10 is periodically returned to the first survey pointP1 it will effect a total error vector 32, like that shown for the fifthsurvey point P5 in FIG. 2 where P5 may also represent return to thefirst survey point P1.

By resolving the total error vector 32 based on the duration of thesurvey, and based on actual time intervals measured between thesequential survey points, corresponding error corrections can be made ateach of the intervening survey points between the first point P1, andthe return thereto.

A similar correction in drift error can be performed by establishing thetotal drift error vector from the initial reference location P1 and anysubsequent reference location, such as fifth point P5, which can have aseparately determined known position, just as the first referencelocation P1 is assumed to have the known (0,0,0) reference coordinates.

By understanding and knowing the form of the specific drift error overtime, various corrections therefor may be mathematically effected insuitable software in the laptop computer 14 to suitably correct themeasured survey coordinates X,Y,Z at each of the survey points duringwhich the drift error is experienced.

Another method for correcting drift error of IMUs includes suitablyintroducing a zero velocity (V=0) update in the integration ofacceleration as conventionally known. Since a significant component ofdrift error is attributed to the double integration of acceleration toobtain displacement in the IMU, periodically introducing the zerovelocity update restarts a portion of the integration process whichestablishes displacement along the three linear coordinates X,Y,Z.

Although, the concept of zero velocity update is conventionally known,there are different methods of introducing such update, yet again havingdifferent advantages and problems.

In view of the special hand-held configuration of the metroprobe 10illustrated in FIGS. 1 and 2, it may be periodically placed in astationary cradle 34 at one or more of the survey points Pn, upon whicha zero velocity (V=0) may then be introduced to update the IMU 18 andreduce the drift errors in all three linear coordinates for subsequentsurvey points.

Since the metroprobe 10 is typically hand-held during the surveyprocess, it is difficult to actually hold still to establish in realityzero velocity thereof.

The cradle 34 can be specifically configured to provide a complementaryseat or socket for temporarily rigidly locking therein the metroprobe 10to ensure that introduction of the zero velocity update into theprocessor controlling IMU operation occurs in fact when the metroprobe10, and integrated IMU 18, are in fact stationary with zero movement orvelocity.

In FIGS. 1 and 2, the cradle 34 can be temporarily affixed or hot-gluedat any suitable location for conducting the survey and/or introducingthe zero velocity update.

For example, FIG. 2 illustrates schematically that the cradle 34 can beaffixed in the object 12 initially at the first survey point P1 toensure an accurate initial calibration of the IMU 18 for the referenceor starting coordinates X,Y,Z, at which the IMU 18 will have no movementor motion, and therefore should record an accurate reference coordinatelocation.

FIG. 2 also illustrates that another cradle 34 can be affixed to theobject 12 at the exemplary fifth survey point P5, at which themetroprobe 10 will again be held stationary with no movement or motion,or velocity, and therefore the zero velocity (V=0) update can beaccurately introduced in the IMU processor so that drift errors will betemporarily reduced, after which the drift errors will again(re)accumulate as the survey continues.

During the measurement survey, the metroprobe 10 is transported in turnto each of the survey points Pn at which the record button 30 isdepressed for recording in the laptop computer 14 the six IMUcoordinates X,Y,Z,A,B,C for each of the survey points. This surveyprocess is repeated for each survey point, and depending on the durationof the survey, and the need for zero velocity update, such update may beintroduced as desired or required.

And, if desired, the cradles 34 may be temporarily affixed or hot-gluedat each of the intended survey points Pn for temporarily affixingthereat the portable metroprobe 10 to ensure no movement thereof whenthe coordinate measurements are being made.

Alternatively, the user may be instructed to manually hold still themetroprobe 10, without using the cradle, at various survey points Pnwithin the physical ability to do so to minimize movement of themetroprobe 10 when the record button 30 is depressed to maximizeaccuracy of the recorded position Pn.

At the end of the survey, or at convenient intervals therein, themetroprobe 10 may be returned to the original reference point, P1 forexample, for subsequently establishing a total or interim error vector,which may then be resolved as disclosed above for suitably correctingthe measured survey coordinates X,Y,Z for the respective survey pointsPn.

Error correction and introduction of zero velocity updates can beapplied either singly or in combination as desired for any particularmeasurement survey; and may also be applied at suitable intervals inperforming the survey depending upon the expected duration of thesurvey.

However, measurement precision in the metrology survey can be furtherimproved by using a modified metroprobe system 36 for performing largevolume dimensional metrology of the object 12 as initially shown in FIG.4.

The metroprobe system 36 includes a suitable coordinate measurementmachine (CMM) in the exemplary form of a conventional laser tracker 38having a variable horizontal (H) and vertical (V) field of view throughwhich a laser beam 40 is aimed or directed toward the various surveypoints Pn for accurately measuring the distance D thereto.

For example, one suitable laser tracker 38 is the FARO Laser TrackerION™ commercially available from distributors for FARO Technologies Inc,Lake Mary, Fla., and has a horizontal field of view of +/−270°; and avertical field of view of +75° and −50°, with an exemplary accuracy orprecision of about 15 microns (0.0006 inches) at 18 meters.

Another example of the laser tracker 38 is the Leica Absolute TrackerAT402 commercially available from distributors for Leica Geosystems,Norcross, Ga., and has a horizontal field of view of +/−360°; and avertical field of view of +/−145°, with an exemplary accuracy orprecision of about 15 microns (0.0006 inches) per meter for the measureddistance.

Laser trackers typically operate with a cooperating reflective target inthe exemplary form of a Spherically Mounted Retro-reflector (SMR).

Accordingly, the portable metroprobe 10 illustrated in FIG. 4 ispreferably modified so that the inertial measurement unit (IMU) 18 isintegrated with both the elongate probe 20 as previously described, anda spherically mounted retro-reflector (SMR) target 42 specificallyconfigured to reflect back to the laser tracker 38 the laser beam 40 foraccurately measuring the distance D therebetween.

The SMR target 42 may have any conventional configuration, and istypically commercially available in paired or matched configuration withthe specific laser tracker, such as the FARO or Leica examples presentedabove, for maximizing accuracy of measurement. Fundamentally, the SMRtarget includes a precision reflector for reflecting back the laser beamto the laser tracker for precisely measuring the distance therebetween.

The SMR reflector is typically configured as a corner cube having threeorthogonal mirrors joined together at a common target corner from whichthe laser beam 40 is reflected back to the laser tracker 38 formeasuring the distance D thereto.

The spherical coordinates H,V for the laser beam 40 and the measureddistance D to the matched target 42 may then be resolved or converted tocorresponding linear coordinates X,Y,Z to define the 3D position orlocation of the target 42 based on a suitable reference location.

The laptop controller 14 is operatively joined to both the laser tracker38 and the IMU 18 for controlling operation thereof.

As shown in FIG. 4, any suitable communication between the controller 14and the laser tracker 38 and IMU 18 may be used, such as a wired tether,or wireless communication using standard WLAN adapters 44 integratedtherewith using standard input/output ports as shown schematically inFIG. 5.

The laptop controller 14 is suitably configured in software to controloperation of the laser tracker 38 and IMU 18 in conducting themeasurement survey, and in particular is used to correlate position andattitude of both the probe 20 and the target 42 to the referenceX,Y,Z,A,B,C Cartesian coordinate system in the IMU 18.

The laser tracker 38 is configured in conjunction with the metroprobe 10for establishing position coordinates for the probe 20 at the pluralityof survey locations or points Pn on the object 12 based on coordinatelocation X,Y,Z of the target 42 as measured by the laser tracker 38 andbased also on attitude A,B,C of the IMU 18 as measured by the IMU 18.

A modified method of performing large volume dimensional metrology onthe object 12 may therefore include measuring linear positions X,Y,Z ofthe IMU 18 by the independent coordinate measurement machine (CMM)having direct line-of-sight (LOS) with the metroprobe 10; and thenestablishing the linear coordinates X,Y,Z of the probe tip 22 at thecorresponding survey points Pn as correlated to the linear positionsX,Y,Z of the IMU 18 as measured by the CMM.

In a preferred configuration, the CMM comprises the laser tracker 38having the variable horizontal (H) and vertical (V) field of view, andthe cooperating target 42 is integrated with both the IMU 18 and theprobe 20 in the common metroprobe 10.

As shown in FIG. 5, the target 42 has a second offset length L2 asmeasured between its reflective target corner and the origin 26 of thecommon X,Y,Z,A,B,C Cartesian coordinate system in the IMU 18. Theposition of the target 42 at its target corner, as measured by the lasertracker 38, is similarly correlated to the coordinate system origin 26in the IMU 18 based on the A,B,C attitude measurement of the IMU 18 forcorrespondingly establishing the linear position X,Y,Z of the IMU 18,and in turn establishing the linear coordinates X,Y,Z of the probe tip22 at the various survey points Pn.

The IMU 18 provides a common reference for correlating movement of boththe probe tip 22, as described above, and the attached target 42 duringoperation based on the measured attitude A,B,C of the IMU 18.

In the autonomous embodiment of the metroprobe 10 described above, theposition of the probe tip 22 is correlated to the position of the commonorigin 26 as measured by the IMU 18.

In the SMR target 42 modification of the metroprobe 10, the moreaccurate location of the target 42 as measured by the laser tracker 38is substituted for the less accurate location of the IMU origin 26 asmeasured by the IMU itself, and a similar correlation is used betweenthe target 42 and the probe tip 22 but still based on the common origin26 of the integrated IMU.

In the exemplary configuration illustrated in FIG. 5, the probe 20 andIMU 18 are coaxially aligned with the common Z-axis in the sameconfiguration illustrated in FIG. 2, but the SMR target 42 is furtherintroduced in a special configuration additionally coaxially alignedwith the common Z-axis. In this special configuration, the measuredcorner reflector of the target 42 and origin 26 of the IMU 18 and theprobe tip 22 are all coaxially aligned in a straight line having a totallength of L+L2.

In this modified correlation, the linear coordinates of the probe tip 22may be generally expressed as P22(XYZ)=P42(XYZ)+V26(XYZ)+V22(XYZ); whereP42(XYZ) represents the measured coordinates of the target 42, V26(XYZ)represents the coordinate vector from the measured target 42 to the IMUorigin 26, and V22(XYZ) again represents the coordinate vector from theorigin 26 to the probe tip 22.

The tip vector V22 is the same as that described above.

The origin vector V26 can be similarly resolved by trigonometry forobtaining the respective components of the target offset length L2 fromthe measured target 42 as represented by L2(XYZ) which is yet again thesame function of attitude A,B,C of the IMU 18 and probe 20.

In the special configuration of the coaxially aligned target 42, IMU 18,and probe 20, changes in attitude in the yaw C-axis do not affect thecoordinates of the probe tip 22 and simplifies the trigonometry.

For the exemplary vertical attitude A,B,C=(0,0,0) of the metroprobe 10shown at survey point P4 to the right in FIG. 5, the coordinatesP22(XYZ) of the probe tip 22 are simply the measured coordinatesP42(XYZ) of the target 42 minus the total offset lengths L2+L, orP22(XYZ)=(X,Y,Z−(L2+L)), which represents the linear coordinates ofmeasured survey point P4.

In another exemplary vertical attitude of the metroprobe 10 shown atsurvey point P3 in FIG. 5, the metroprobe 10 again has an attitudeinclination angle dB in the −B pitch direction only, i.e.(A,B,C)=(0,−dB,0), which again inclines counterclockwise the metroprobe10 solely in the X-Z plane, and correlates with the new coordinateposition of the probe tip P22(XYZ)=(X+(L2+L)×Sine(dB), Y,Z−(L2+L)×Cosine(dB)) relative to the new P42(XYZ) coordinate position ofthe target 42 as measured by the laser tracker 38. The probe tip 22contacts the third survey point P3 whose coordinate position thereforematches the so calculated new tip coordinates P22(XYZ).

The vectors V26 and V22 may be similarly resolved in the YZ plane forany roll inclination angle dA in the roll rotary axis-A, or in anyattitude corresponding with the changing attitude of the metroprobe 10during operation.

In other configurations of the metroprobe 10 in which the target 42, IMU18, and probe tip 22 are not coaxially aligned, corresponding vectorsV26 and V22 still define the corresponding offset length L2 between thetarget 42 and origin 26, and offset length L between the origin 26 andprobe tip 22, and similar vector analysis may be used to correlatecoordinate location P22(XYZ) of the probe tip 22 to the origincoordinates P26(XYZ) and target coordinates P42(XYZ).

Accordingly, the linear X,Y,Z coordinate position of the probe tip 22may be mathematically correlated and established by combining the lineartranslation of the metroprobe 10 as measured at the target 42 with theangular attitude of the metroprobe 10 as measured by the IMU 18 in anysuitable orientation or attitude for accessing any survey point,including otherwise hidden survey points.

As described above, the target 42 preferably comprises the sphericallymounted retro-reflector (SMR), which is suitably affixed or mounted atopthe housing 16 containing the IMU 18 in the metroprobe 10. Other thanthe addition of the SMR target 42, the metroprobe 10 is identical inconfiguration and function to the one described above, including the useof the three accelerometers 24 and three gyroscopes 28 forcorrespondingly defining the same three orthogonal linear axes X,Y,Z andthe same three respective angular axes A,B,C.

In the basic measurement survey illustrated in FIG. 5, the laser tracker38 is used to precisely measure linear position of the target 42 bydirecting the laser beam 40 directly along the line-of-sight (LOS)therewith to in turn establish the linear coordinates of the probe tip22 as correlated with the target 42 through the IMU 18 as presentedabove.

By also measuring attitude (A,B,C) of the IMU 18 using the threegyroscopes 28 therein, suitable correlation with attitude of both theprobe 20 and the target 42 having the corresponding offset lengths L,L2may then be used to correlate the linear position of the probe tipP22(XYZ) to the coordinate system origin 26 in the IMU 18 as alsodescribed above.

However, when line-of-sight (LOS) to the target 42 is blocked by someobstruction 46, such as a portion of the survey object 12 itself, thelinear position of the IMU 18 is instead measured using the threeaccelerometers 24 therein to in turn establish the linear coordinates ofthe probe tip P22(XYZ) as correlated with the IMU 18 in the mannerdescribed above.

The survey process may be further modified by placing the metroprobe 10in a stationary cradle 34 at one of the survey points, like point P4shown in FIG. 5, when line-of-sight (LOS) to the target 42 is blocked bythe obstruction 46.

By using the cradle 34 to temporarily immobilize the metroprobe 10during the survey, the zero velocity update (V=0) may be introduced inthe IMU 18 in the same manner described above to reduce drift errors inall three linear coordinates (X,Y,Z) in sequential or subsequent surveypoints in which line-of-sight (LOS) to the target 42 may also beblocked.

IMU performance can be improved by updating it with known positioninformation whenever possible. Since the precise location of the SMRtarget 42 as measured by the laser tracker 38 is continually updatedduring the survey, the correlated and equally precise location of theIMU 18 may also be continually updated by the controller 14.

The introduction of the zero velocity update in the IMU 18 isappropriate whenever the IMU 18 is known to be stationary since thedrift errors continuously accumulate, and the update may be introducedmanually through the controller 14 or manually upon depressing therecord button 30 when the IMU 18 is mounted in the stationary cradle 34.

The zero velocity update may even be introduced by programming thecontroller 14 to detect and recognize in the IMU 18 either measuredacceleration or velocity below a defined low-value threshold, whichthreshold could be a function of the error-time performance as specifiedfor a particular IMU.

A particular advantage of the metroprobe 10 is its configuration andtotal length L2+L as measured from target-to-tip to improve access tovarious survey points Pn, especially those points partially or fullyhidden by various obstructions from the field of view of the lasertracker 38.

Most of the metroprobe 10 can be hidden from LOS access, as long as thetarget 42 remains visible within the LOS of the laser tracker 38, and aprecise measurement of the hidden survey point, such as point P3, maystill be made. The precision or accuracy of the measured coordinates atthe probe tip P22(XYZ) will closely match the specified precision of thelaser tracker 38 itself due to the low bias error and high precisionoperation of the gyroscopes 28 in the IMU 18 which are used to correlateposition of the probe tip 22 to the position of the target 42 beingmeasured.

When the target 42 itself is hidden from the laser tracker 38, thecoordinate position P26(XYZ) of the IMU origin 26 is instead measuredusing the IMU 18 itself and correlated to the coordinate position of theprobe tip 22, having a measurement accuracy dependent on accuracy of theIMU 18, including the substantial accuracy component due to the low gyrobias errors inherent therein.

Additional advantages may accrue to the combined use of the lasertracker 38, IMU 18, and controller 14 as specifically configured forcontrolling operation thereof.

As described above, linear position P26(XYZ) of the IMU 18 at its origin26 may be measured by the IMU using its accelerometers 24, and similarlyreverse-correlated to the position P42(XYZ) of the SMR target 42 so thatmeasured location of the IMU 18 may be specially used to determinecoordinate location P42(XYZ) of the target, independent of the locationof the target as measured by the laser tracker 38.

In this way, the laser tracker 38 may itself be feedback-controlled tofollow or track movement of the target 42 atop the metroprobe 10 basedon the linear coordinate position P26(XYZ) of the IMU 18 as measured bythe IMU itself.

The IMU 18 provides a new ability for obtaining automatic tracking(Auto-Tracking) between the laser tracker 38 and its SMR target 42within the specified field-of-view of the laser tracker. And, thisautomatic tracking may be accomplished using any conventional SMR targetin its simplest and most inexpensive fixed form.

However, further improvements may be obtained by using a target 42 m asillustrated in FIG. 4 in the special configuration of a multiaxismotorized SMR pivotally mounted atop the IMU 18 in the metroprobe 10.

Any conventional motorized SMR may be used, such as the Active Target™commercially available through distributors for Automated Precision Inc,Rockville, Md. The SMR Active Target 42 m has an azimuthal trackingangle corresponding with the yaw C-axis of unlimited 360°, and anelevational tracking angle corresponding with the pitch A-axis of +80°and −55°. The SMR may therefore be actively directed within its field ofview toward the laser tracker source of the laser beam incident to themotorized SMR.

The laser tracker 38, the motorized SMR target 42 m, and the IMU 18 areall operatively joined to the common controller 14 which is suitablyconfigured in software for automatically guiding or tracking the lasertracker 18 to actively follow movement of the motorized SMR target 42 m,and also automatically back-tracking the motorized SMR target 42 m toactively follow heading or direction movement of the laser tracker 38 inresponse to IMU-correlated position of the SMR target 42 relative toposition and attitude of the IMU 18.

Automatic tracking between a laser tracker and its SMR target is acommon feature that ensures that the laser beam of the tracker iscontinuously aimed at the target as the target itself moves during ameasurement survey.

Using a non-motorized target, the laser tracker itself must be suitablymotorized and controlled to aim its laser beam toward the moving target.

Using a motorized target, both motorized target and laser tracker canimprove automatic tracking therebetween, but such operation stillrequires LOS visibility therebetween.

By further introducing the IMU 18 in the metroprobe 10, furtherimprovement and tracking accuracy may be additionally obtained bycommunicating aiming directions to the laser tracker 38 to followmovement of the motorized target 42 m based on its location as measuredby the IMU 18.

In this configuration, LOS visibility between the laser tracker 38 andSMR target 42 m can be temporarily lost or broken by variousobstructions during the survey, but the laser tracker 38 cannevertheless still be controlled to still follow the temporarily hiddenSMR target 42 m.

By maintaining continuous and accurate tracking of the laser beam 40emitted from the laser tracker 38 and either its non-motorized SMRtarget 42 or motorized SMR target 42 m, the measurement survey can becompleted more accurately and with minimal, if any, interruptions, andthereby enhance the advantages associated with active tracking of theSMR target.

Further improvements in the large volume dimensional metrology surveymay also be obtained by optionally integrating a second IMU 48 with thelaser tracker 38 itself as shown in FIG. 4. The second IMU 48 may haveany conventional configuration, such as the exemplary configurationsdescribed above for the first IMU 18 integrated in the metroprobe 10.

The second IMU 48 is fixedly attached to the laser tracker 38 and againsuitably operatively joined to the common laptop computer controller 14for measuring linear position (XYZ) and attitude (ABC) of the second IMU48.

The measurement survey may then be conducted using the IMU-embeddedlaser tracker 38 at two different reference locations havingline-of-sight with a plurality of common survey points for measuringcoordinates thereof.

The least squares iterative optimization process introduced above in theBackground section may then be conducted using the position and attitudeof the second IMU 48 at the two different reference locations and themeasured coordinates at the common survey points to conform allmeasurements from the laser tracker to a common coordinate referencesystem.

The mathematical least squares process may be used whenever appropriateto improve CMM measurement accuracy and confidence anytime multiple CMMobservations occur on a measurement or common tie-in point tore-establish a common coordinate reference system.

The mathematical optimization process to tie-in all the measurementsfrom all the different CMM locations is generally referred to inacademia as a least squares problem. There are numerous variations ofthe tie-in process. However, least squares is the underlyingmathematical principal.

When least squares is used in the metrology process, each measurementpoint can be treated as three variables. For example, the Cartesianlocation X, Y, and Z. Also, each CMM can be treated as six variables.For example, the Cartesian position of the CMM X, Y, and Z and its threeorientation angles pitch (A), roll (B), and yaw (C).

The actual measurement data from each CMM to each point can be treatedas up to three observations. In the case of the CMM laser tracker, theobservations can be laser range, horizontal laser pointer angle, andvertical laser pointer angle.

After initial estimates of the CMM X,Y,Z position and A,B,C orientationare created, all the variables and observations for each measurement arewritten out in a system of equations and solved simultaneously in aleast squares fashion. The least squares solution produces adjustmentsto the three measurement point variables and six CMM variables.

The adjustments should result in a better least squares fit for each ofthe measurement observations. The measurement observations areconsidered constants in the system of equations. After applying theadjustments to the variables, a new system of simultaneous equations isconstructed and solved for another set of adjustments for the samevariables.

The iterative process is repeated until the adjustments to the variablesare insignificant and the optimum measurement point locations and CMMpositions and orientations are obtained in a common reference systemtying together all measurement points and the two or more CMM viewinglocations.

This mathematical least squares process is merely a general description.Variations of the least squares process can be effectively applied toaccommodate for scale adjustments, constraints on various measurements,confidence weighting on various measurements, and many other issues inaccordance with conventional practice.

The first step in the mathematical optimization process provides aninitial estimate of the position and orientation of the CMM at itsdifferent locations in the measurement survey. Conventionally, thisinitial estimate is arbitrary, and may be randomly selected.

By embedding the second IMU 48 in the CMM and aligning it with a commonposition and orientation, the second IMU 48 can provide a generallyaccurate position and orientation for the CMM at the different locationsfor the initial step of the mathematical optimization process forimproving that process.

Aligning the second IMU 48 in the CMM to a common position andorientation can be effected for one or more CMMs used in the measurementsurvey. When a single CMM is being used for the metrology survey,position and orientation of the single CMM on the first measurement ortie-in point observation may be used as the reference. When the CMM ismoved for observations on a different measurement or tie-in point, thesecond IMU 48 embedded in the CMM can report its new position andorientation parameters for utilization in the least squares process.

For the case of multiple CMMs, the mathematical least squaresoptimization process may begin with simple least squares estimates ofthe CMM position and orientation. However, after observations for eachCMM on common points are taken, the complete least squares process canbe used to update the position and orientation of the second IMU 48 ineach CMM for subsequent measurements.

The IMU updating procedure just described could precede the metrologysurvey as a calibration phase. The IMU updates could be repeatedthroughout the survey anytime a complete least squares optimizationprocess is desired and accomplished.

Fundamental to the various improvements in the large volume dimensionalmetrology measurement survey described above in various configurationsis the common use of the special metroprobe 10 which integrates the IMU18 with a correlated touch probe 20. The IMU itself may be used forconstantly and accurately monitoring the angular attitude of the IMU andattached probe 20 within the high accuracy of the gyroscopes whichtypically have very low gyro bias drift error irrespective of the gradeand cost for the IMU.

Such metroprobe 10 provides convenient access to various survey points,some of which might be hidden from the typical line-of-sight requirementfor typical CMMs.

In the most basic configuration, the accelerometers in the IMU are usedin an autonomous mode of operation of the metroprobe for measuring thelinear coordinates X,Y,Z of the probe tip as precisely correlated to thereference origin in the IMU, and within the accuracy attributable to theaccelerator drift error bias as specified for the IMU.

In an enhanced configuration, the metroprobe 10 is integrated with a CMMin the preferred form of the laser tracker 38 for precisely measuringlocation of the cooperating target 42, which is precisely correlated tolocation of the probe tip 22 using the measured attitude of the IMU forin turn precisely measuring location of various survey points, with lessregard to partially or fully hidden ones thereof.

Various features of the measurement survey process and apparatustherefor have been disclosed above, and may be used in variouscombinations and configurations consistent therewith, based onrecombining any one or more of the individual features presented in thefollowing claims which are merely representative and not limiting.

While there have been described herein what are considered to bepreferred and exemplary embodiments of the present invention, othermodifications of the invention shall be apparent to those skilled in theart from the teachings herein, and it is, therefore, desired to besecured in the appended claims all such modifications as fall within thetrue spirit and scope of the invention.

Accordingly, what is desired to be secured by Letters Patent of theUnited States is the invention as defined and differentiated in thefollowing claims in which we claim:

1. A method of performing dimensional metrology of an object comprising:integrating an Inertial Measurement Unit (IMU) with an elongate probe ina portable metroprobe; correlating position of a tip of said probehaving a first offset length from an origin of a coordinate system insaid IMU based on attitude measurement of said IMU; and transportingsaid metroprobe to a plurality of survey points on said object formeasuring corresponding coordinates thereof based on measured attitudeof said IMU.
 2. A method according to claim 1 wherein: said IMUcomprises a plurality of accelerometers and gyroscopes forcorrespondingly defining three orthogonal linear axes and threerespective angular axes; and linear position of said probe tip iscorrelated to said coordinate system origin based on angular attitude ofsaid IMU as measured by said gyroscopes.
 3. A method according to claim2 further comprising transporting said metroprobe to a referencelocation to establish at said probe tip three linear referencecoordinates relative to said three linear axes and three angularreference coordinates relative to said three angular axes.
 4. A methodaccording to claim 2 wherein said probe tip offset position iscorrelated to said coordinate system origin based on angular position ofsaid probe relative to at least one of said three linear axes; and saidmetroprobe is operable with attitudes ranging from horizontal tovertical.
 5. A method according to claim 1 further comprising:transporting said metroprobe in a sequential survey of said surveypoints; measuring linear positions of said IMU at each of said surveypoints; measuring attitude of said IMU to correlate therewith attitudeof said probe; and establishing corresponding linear coordinates of saidprobe tip in contact with said survey points correlated to said measuredlinear positions of said IMU.
 6. A method according to claim 5 wherein:said IMU comprises a plurality of accelerometers and gyroscopes forcorrespondingly defining three orthogonal linear axes and threerespective angular axes; said angular attitude of said IMU is measuredby said gyroscopes in said IMU; and said linear positions of said IMUare measured by both said accelerometers and said gyroscopes in saidIMU.
 7. A method according to claim 6 wherein said metroprobe isautonomous in performing said dimensional metrology of said object, andis operatively joined to a controller for establishing said linearcoordinates of said probe tip at said survey points relative to saidthree orthogonal linear axes based on said measured linear position andattitude of said IMU.
 8. A method according to claim 7 wherein said IMUis subject to drift errors, and said linear coordinates at said surveypoints are corrected to reduce said drift errors.
 9. A method accordingto claim 7 further comprising: transporting said metroprobe between tworeference locations in surveying said survey points; establishing aposition error vector based on a difference in linear positions measuredat said two reference locations; and resolving said error vector toreduce corresponding errors in said linear coordinates at said surveypoints.
 10. A method according to claim 9 wherein: said IMU drift errorsincrease with time during transportation of said metroprobe during saidmeasurement survey; and said error vector is resolved to correspondinglyreduce more error in said linear coordinates at subsequent surveypoints.
 11. A method according to claim 8 further comprising: placingsaid metroprobe in a stationary cradle at one of said survey points; andintroducing a zero velocity update in said IMU to reduce said drifterrors in all three linear coordinates for subsequent survey points. 12.A method according to claim 5 further comprising: measuring linearpositions of said IMU by an independent coordinate measurement machine(CMM) having line-of-sight with said metroprobe; and establishing saidlinear coordinates of said probe tip at said survey points correlated tosaid linear positions of said IMU measured by said CMM.
 13. A methodaccording to claim 12 wherein: said CMM comprises a laser tracker havinga variable horizontal and vertical field of view; a cooperating targetis integrated with said IMU and said probe in said metroprobe, and has asecond offset length from said origin of said coordinate system in saidIMU; and position of said target as measured by said laser tracker iscorrelated to said coordinate system origin in said IMU based on saidattitude measurement of said IMU for correspondingly establishing saidlinear position of said IMU; and in turn establishing said linearcoordinates of said probe tip at said survey points.
 14. A methodaccording to claim 13 wherein: said target comprises a sphericallymounted retro-reflector (SMR) affixed atop said IMU in said metroprobe;and said IMU comprises a plurality of accelerometers and gyroscopes forcorrespondingly defining three orthogonal linear axes and threerespective angular axes.
 15. A method according to claim 14 furthercomprising: measuring linear position of said target using said lasertracker having line-of-sight therewith to in turn establish said linearcoordinates of said probe tip as correlated with said target throughsaid IMU; and measuring linear position of said IMU using saidaccelerometers therein when line-of-sight to said target is blocked toin turn establish said linear coordinates of said probe tip ascorrelated with said IMU.
 16. A method according to claim 14 furthercomprising measuring attitude of said IMU using said gyroscopes thereinto correlate with attitude of both said probe and said target havingcorresponding offset lengths to correlate said linear position of saidprobe tip to said coordinate system origin in said IMU.
 17. A methodaccording to claim 14 further comprising: placing said metroprobe in astationary cradle at one of said survey points when line-of-sight tosaid target is blocked; and introducing a zero velocity update in saidIMU to reduce drift errors in all three linear coordinates in sequentialsurvey points in which line-of-sight to said target is also blocked. 18.A method according to claim 14 further comprising: operatively joiningsaid laser tracker and said IMU to a controller; measuring linearposition of said IMU using said accelerometers therein; correlatingposition of said SMR target to said measured linear position of saidIMU; and controlling said laser tracker to follow movement of saidtarget atop said metroprobe based on said measured linear position ofsaid IMU.
 19. A method according to claim 14 wherein: said targetcomprises a multiaxis motorized SMR pivotally mounted atop said IMU insaid metroprobe; and said laser tracker, said SMR target, and said IMUare operatively joined to a controller for automatically tracking saidlaser tracker to said SMR target and automatically back-tracking saidSMR target to said laser tracker in response to correlated position ofsaid SMR relative to position and attitude of said IMU.
 20. A methodaccording to claim 14 further comprising: integrating a second IMU withsaid laser tracker for measuring position and attitude of said secondIMU; conducting said survey using said laser tracker at two differentreference locations having line-of-sight with a plurality of commonsurvey points for measuring coordinates thereof; and performing a leastsquares iterative optimization process using said position and attitudeof said second IMU at said two different reference locations, and saidmeasured coordinates at said common survey points to conform allmeasurements from said laser tracker to a common coordinate referencesystem.
 21. A system for performing dimensional metrology of an objectcomprising: a coordinate measurement machine laser tracker having avariable horizontal and vertical field of view; a portable metroprobeincluding an inertial measurement unit integrated with both an elongateprobe and a spherically mounted retro-reflector target; a controlleroperatively joined to said laser tracker and said IMU for controllingoperation thereof; said controller being configured to correlateposition and attitude of both said probe and said target to a referencecoordinate system in said IMU; and said laser tracker being configuredin conjunction with said metroprobe for establishing coordinates at saidprobe at a plurality of survey locations on said object based onlocation of said target as measured by said laser tracker and based alsoon attitude of said IMU as measured by said IMU.